Ventricle assist devices are frequently used to boost blood circulation to assist a heart which still functions but is not pumping sufficient blood for adequate circulation. The estimated need for a reliable long-term ventricle assist device (VAD) is presently projected at between 50,000 and 100,000 patients per year in the U.S. alone.
At the present time, rotary blood pumps are often the preferred type of pump for use as a ventricle assist device as compared to other more complex types of pumps which may use pistons, rollers, diaphragms, compliance chambers, and so forth. This is at least partially because rotary pumps may be manufactured in larger numbers at a relatively lower cost and are typically less complex than other types of pumps. The more complex pumps, on the other hand, may cost up to $50,000 per unit Furthermore, availability of large numbers of complex pumps, as is required by the sizeable population that could benefit from such pumps, is limited by high manufacturing, operating, and maintenance costs. Therefore, rotary blood pumps are increasingly used not only for ventricular assist applications, but also for cardiopulmonary bypass procedures and percutaneous cardiopulmonary support applications in emergency cases.
Clinical uses of rotary pumps are normally limited to a few days owing to shortcomings of the devices. A non-comprehensive list of such problems or shortcomings would include the following: (1) blood damage which may occur when blood comes into contact with rotor bearings due to bearing heat or being forced through small clearances, (2) the need for bearing purge systems which may require percutaneous (through the skin) saline solution pump systems, (3) bearing seizure resulting from the considerable thrust and torque loads, or from dried blood sticking on the bearing surfaces, (4) problems of blood damage (hemolysis) and blood clotting (thrombosis) caused by relative rotational movement of the components of the pump, (5) pump and control size and shape limitations necessary for implantation or convenient mobility, (6) weight limitations for implantation to avoid tearing of implant grafts due to inertia of sudden movement, (7) difficulty in coordinating and optimizing the many pump design parameters which may affect hemolysis, (8) high power consumption that requires a larger power supply, (9) motor inefficiency caused by a large air gap between motor windings and drive magnets, (10) heat flow from the device to the body, (11) complex Hall Effect sensors/electronics for rotary control, (12) the substantial desire for minimizing percutaneous (through the skin) insertions including support lines and tubes, (13) large pump and related hose internal volume which may cause an initial shock when filled with saline solution while starting the pump, and other problems.
Existing bearing systems for externally used rotary blood pumps may have small rolling element bearings such as ball bearings. Rolling element bearings require a shaft seal to prevent blood entering the bearing voids between the rolling elements. If blood enters the bearing voids, it coagulates and may cause bearing seizure by interfering with the rolling elements. Shaft seals complicate pump design, decrease pump reliability, and reduce pump life.
Some implantable blood pumps utilize pivot bearings. Pivot bearings can operate immersed in blood without a blood seal. However, to maintain the precise rotation required in blood pumps to minimize red blood cell damage, such pivot bearings utilize complicated shaft pre-load mechanisms to eliminate shaft end-play. Shaft pre-load mechanisms are prone to seizure by coagulated blood. They also increase bearing wear.
Other blood pump bearing systems utilize journal bearings flushed with fluids such as saline solution or blood. Journal bearings have minimal wear, but require a separate thrust bearing that complicates pump design. Journal bearings require fluid pressure to support the loads. If the pump utilizes saline solution rather than blood as the bearing fluid, then pump design is significantly complicated by the need for a separate reservoir, flow lines, and the like. If the pump utilizes blood as bearing fluid, then potential pump seizure caused by coagulated blood is a serious concern. In addition, blood flow through a journal bearing is exposed to a high shear environment. The high shear environment may damage the blood or generate micro-clots that are washed into the patients blood stream. Finally, journal bearings of the size used in blood pumps require very precise alignment that increase manufacturing complexity, and increase costs.
Although a significant amount of effort has been applied to solving the problems associated with rotary pumps, there is still a great demand for a safe, reliable, and durable blood pump that may be used for longer term applications.
The following patents describe attempts made to solve problems associated with rotary blood pumps including ventricle assist devices.
U.S. Pat. No. 4,625,712 to R. K. Wampler discloses a full-flow cardiac assist device for cardiogenic shock patients which may be inserted into the heart through the femoral artery and driven via flexible cable from an external power source. A catheter attached to the pump supplies the pump bearings with a blood-compatible purge fluid to prevent thrombus formation and introduction of blood elements between rotating and stationary elements. Due to the very small diameter of the pump, rotational speeds on the order of 10,000 to 20,000 rpm are used to produce a blood flow on the order of about four liters per minute.
U.S. Pat. No. 4,957,504 to W. M. Chardack discloses an implantable blood pump for providing either continuous or pulsatile blood flow to the heart. The pump includes a stator having a cylindrical opening, an annular array of electromagnets disposed in a circle about the stator concentric with the cylindrical opening, a bearing carried by the stator and extending across the cylindrical opening, and a rotor supported by the bearing. The rotor is in the form of an Archimedes screw and has a permanent magnet in its periphery which lies in the same plane as the circular array of electromagnets and is driven in stepper motor fashion.
U.S. Pat. No. 4,944,722 to J. W. Carriker discloses a percutaneously insertable intravascular axial flow blood pump with a rotor extension and drive cable fitting so designed that the thrust bearing surfaces of the purge seal and cable fitting can be pre-loaded.
U.S. Pat. No. 4,817,586 to R. K. Wampler discloses an intravascular flow blood pump with reduced diameter having blood exit apertures in the cylindrical outside wall of the pump housing between the rotor blades and the rotor journal bearing.
U.S. Pat. No. 4,908,012 to Moise et al. discloses an implantable ventricular assist system having a high-speed axial flow blood pump. The pump includes a blood tube in which the pump rotor and stator are coaxially contained, and a motor stator surrounding the blood duct. A permanent magnet motor rotor is integral with the pump rotor. Purge fluid for the hydrodynamic bearings of the device and power for the motor are preferably percutaneously introduced from extra-corporeal sources worn by the patient.
U.S. Pat. No. 4,779,614 to J. C. Moise discloses an implantable axial flow blood pump which includes a magnetically suspended rotor of relative small diameter disposed without bearings in a cylindrical blood conduit Neodymium-boron-iron rotor magnets allow a substantial gap between the static motor armature and the rotor. Magnetically permeable strips in opposite ends of the pump stator blades transmit to Hall sensors variations in an annular magnetic field surrounding the rotor adjacent the ends of the pump stator blades.
U.S. Pat. No. 5,049,134 to Golding et al. discloses a seal free centrifugal impeller supported in a pump housing by fluid bearings through which a blood flow passageway is provided.
U.S. Pat. No. 4,382,199 to M. S. Isacson discloses a hydrodynamic bearing system for use with a left ventricle assist device. The bearings are formed by the fluid in the gap between the rotor and the stator.
U.S. Pat. No. 4,135,253 to Reich et al. discloses a centrifugal blood pump provided with a magnetic drive system which permits a synchronous magnetic coupling with a separate power unit disposed immediately adjacent the pump housing but outside of the skin surface. The pump has a single moving part which includes the combination of an impeller connected to a magnetic drive rotor. The magnetic drive system floats on a fluid surface of saline solution.
U.S. Pat. No. 4,507,048 to J. Belenger discloses a centrifugal blood pump with a bell shaped housing having a suction inlet at the apex and a tangential outlet adjacent the base. A conical rotator is driven by spaced permanent magnets embedded in the base of the rotator and an externally generated rotating magnetic field.
U.S. Pat. No. 4,688,998 to Olsen et al. discloses a pump with a magnetically suspended and magnetically rotated impeller. The impeller may be configured for axial flow with a hollow, cylindrical-type impeller with impeller vanes on the internal surface thereof. The impeller includes a plurality of internally embedded, permanent magnets that cooperate with electromagnets for drive and position control of the impeller.
U.S. Pat. No. 4,763,032 to Bramm et al. discloses a magnetic rotor bearing for suspending a rotor for an axial or radial-centrifugal blood pump in a contact-free manner, and comprising a permanent and electromagnetic arrangement.
U.S. Pat. No. 4,846,152 to Wampler et al. discloses a miniature high-speed intravascular blood pump with two rows of rotor blades and a single row of stator blades within a tubular housing. The rotor's first row has no provision for a variable pitch blade but produces a mixed centrifugal and axial flow by increasing hub diameter. The rotor's second row, axially spaced and having an axial distance between the first row, produces a purely axial flow. The stator blades are reverse twisted to straighten and slow the blood flow.
U.S. Pat. No. 4,944,748 to Bramm et al. discloses an impeller in a blood pump supported by permanent magnets on the impeller and pump housing and stabilized by an electromagnet on the housing. The impeller is rotated magnetically and stator coils in the housing are supplied with electric currents having a frequency and amplitude adjusted in relation to blood pressure at the pump inlet.
U.S. Pat. No. 4,994,078 to R. K. Jarvik discloses an electrically powered rotary hydrodynamic pump having motor windings and laminations disposed radially about an annular blood channel and having a motor rotor disposed therein such that an annular blood channel passes through the gap between the motor rotor and the windings.
U.S. Pat. No. 5,055,005 to Kletschka discloses a fluid pump with an electromagnetically driven rotary impeller levitated by localized opposed fluid forces.
In spite of the effort evidenced by the above patents, there remains the need for an improved rotary pump for use as a ventricle assist device that is reliable, compact, requires limited percutaneous insertions, and produces fewer blood damage problems such as hemolysis and thrombosis. A bearing system for an improved rotary pump should reliably and precisely support the rotor for long-term, maintenance free, low friction operation without the need for bearing seals and lubrication. Those skilled in the art will appreciate the present invention which addresses these and other problems.